It's not an action, but rather a metadata option from the Insert Reference dialog. When you select Insert Reference you should see the Common, Item Properties, Workflow Variables, etc. tabs. It is listed in the Common tab.
Sorry, Sal. I would imagine something they are working on. I'm speculating as I'm clearly not on O365, but you may potentially call into the Nintex web services to get this. Although way more than you want, the GetWorkflowHistoryForListItem method would get what you're looking for.
You can store the Task ID in a variable and then query the task list based on that ID and pull the "modified by" column which should contain the person who approved the task.
So I created one workflow variable, moved the Task ID into that and able to print that value in one of the column using Update Item action,now I know the Task ID is there, I added "Office 365 query list" and In the field I set the Task variable, in the Sort option I added Current Item:Modified By option as I don't see any Task property section and move the results in the Result variable I created. Can you confirm what settings should I use here? I don't see any output in the Result. I know i'm doing something wrong here.
You should be querying the "Modified by" column and not the "taskid variable". Use the "taskid variable" to filter the query for when the "taskid variable" is equal to the "ID" of the task list. This will query the task you created and provide last person who modified it aka the approver.
You can find more information on how to use the query list action in the O365 user guide found here: http://help.nintex.com/en-US/O365/Default.htm
Thanks Andrew, the problem I have is in the filter, as I don't see any ID related to Task list, I can see only Item Properties, workflow context and workflow variable. I got the workflow variable in the filter now if i try to set Equals to action in the second filed, I don't have ID of the task list. How do i compare this workflow variable (that is holding the task ID) ?